Valorization strategies for electrodegradation of nitrogenous wastes in sewage

Abstract

The interconversion of N₂ and N-containing compounds is central to the natural nitrogen cycle, one of the most important global biogeochemical cycles, which plays a crucial role in sustaining life across all organisms. Nitrogen pollution in surface water bodies, caused by the indiscriminate discharge of industrial and domestic wastewater, has become a global environmental concern. The excessive accumulation of nitrogenous wastes poses a serious threat to human health and disrupts the natural nitrogen cycle. Traditional water purification methods, such as chemical redox processes, physical adsorption, and biological treatments, often face limitations, including high energy consumption, low efficiency, large space requirements, prolonged treatment times, sludge generation, and high operating costs. Emerging electrochemical degradation techniques offer promising solutions for efficiently degrading nitrogenous wastes. These electrochemical technologies demonstrate advantages in cost-effectiveness, environmental friendliness, high efficiency, and broad applicability, while also presenting opportunities to generate added value during the electrodegradation processes. Nitrogen-containing wastes in wastewater can be classified into electrophiles (e.g., nitrate and nitrite) and nucleophiles (e.g., ammonia nitrogen, hydrazine, and urea) according to their redox properties. Based on the different properties of nitrogenous wastes, coupling corresponding electrochemical degradation reactions with tailored electrochemical energy storage and conversion devices provides opportunities for additional energy and value generation. Herein, advanced insights into valorization strategies during the electrodegradation processes of representative nitrogenous wastes in sewage are subtly provided, where the approaches for enhanced value output efficiency are highlighted, including (i) coupling the electroreduction of electrophilic pollutants with Zn-electrophile batteries to achieve energy output and simultaneous chemical production, (ii) coupling electro-oxidation of nucleophilic pollutants with hybrid direct fuel cells to realize energy output, (iii) applying hybrid water electrolysis systems assisted with nucleophilic wastes for energy-saving and clean H₂ production, (iv) assembling Zn-nucleophile batteries for energy storage and hydrogen production, and (v) producing valuable chemicals via C-N coupling processes. The cell design, coupled with selection criteria and optimizing strategies of advanced electrodes and cell configuration, is highlighted. Finally, an in-depth analysis of current challenges and future prospects is provided to deepen the understanding of advanced electrochemical cells and bridge the gap between experimental trials and practical applications with respect to mechanism investigation, electrode design and evaluation, and cell design.